Soft x-ray optics with improved filtering

ABSTRACT

Optical elements that efficiently propagate x-ray radiation over a desired energy range and reject radiation outside the desired energy range are presented herein. In one aspect, one or more optical elements of an x-ray based system include an integrated optical filter including one or more material layers that absorb radiation having energy outside the desired energy band. In general, the integrated filter improves the optical performance of an x-ray based system by suppressing reflectivity within infrared (IR), visible (vis), ultraviolet (UV), extreme ultraviolet (EUV) portions of the spectrum, or any other undesired wavelength region. In a further aspect, one or more diffusion barrier layers prevent degradation of the integrated optical filter, prevent diffusion between the integrated optical filter and other material layers, or both. In some embodiments, the thickness of one or more material layers of an integrated optical filter vary over the spatial area of the filter.

TECHNICAL FIELD

The described embodiments relate to x-ray optics, and more particularlyto thin film optical layers employed to filter out of band radiation inoptical systems.

BACKGROUND INFORMATION

Semiconductor devices such as logic and memory devices are typicallyfabricated by a sequence of processing steps applied to a specimen. Thevarious features and multiple structural levels of the semiconductordevices are formed by these processing steps. For example, lithographyamong others is one semiconductor fabrication process that involvesgenerating a pattern on a semiconductor wafer. Additional examples ofsemiconductor fabrication processes include, but are not limited to,chemical-mechanical polishing, etch, deposition, and ion implantation.Multiple semiconductor devices may be fabricated on a singlesemiconductor wafer and then separated into individual semiconductordevices.

Metrology processes are used at various steps during a semiconductormanufacturing process to detect defects on wafers to promote higheryield. A number of metrology based techniques including scatterometryand reflectometry implementations, and associated analysis algorithmsare commonly used to characterize critical dimensions, film thicknesses,composition and other parameters of nanoscale structures.

Traditionally, scatterometry critical dimension measurements areperformed on targets consisting of thin films and/or repeated periodicstructures. During device fabrication, these films and periodicstructures typically represent the actual device geometry and materialstructure or an intermediate design. As devices (e.g., logic and memorydevices) move toward smaller nanometer-scale dimensions,characterization becomes more difficult. Devices incorporating complexthree-dimensional geometry and materials with diverse physicalproperties contribute to characterization difficulty.

Metrology systems based on x-ray based scatterometry and reflectometrymeasurements have emerged as suitable tools for dimensional metrology ofsemiconductor structures. X-ray based metrology systems have exhibitedmeasurement capability for both low and high aspect ratio structures. Insome applications, x-ray based metrology systems feature an illuminationbeam spot size compatible with scribe-line targets. X-ray basedmetrology systems have made it possible to efficiently develop andvalidate measurement recipes for challenging measurement applicationsand operate in a high volume manufacturing (HVM) environment withoutsubstantial prior dimensional and material composition information.

Highly reflective multilayer optics are a critical component of theoptical systems of x-ray based measurement and processing systems.Highly reflective multilayer optics typically employ repeating pairs ofdifferent material film layers. Each pair of layers includes an absorbermaterial layer and a spacer material layer. Common absorber materialsinclude Tungsten (W), Tungsten disilicide (WSi₂), Ruthenium (Ru),Vanadium (V), Lanthanum (La), Molybdenum (Mo), Titanium dioxide (TiO₂),Nickel (Ni), etc. Common spacer materials include Carbon (C), Boronnitride (BN), Boron Carbide (B₄C), Silicon (Si), etc.

FIG. 1 depicts an illustration of a cross-sectional view of a multilayeroptic 10 used in soft x-ray applications. A set of repeated pairs ofmultilayer coatings 12 is fabricated over a Silicon substrate 11. Thetop four repeated pairs of multilayer coatings 13A-D are illustrated.Each repeated pair of multilayer coatings includes a spacer layer (e.g.,layer 15 of repeated pair 13A) and an absorber layer (e.g., layer 14 ofrepeated pair 13A). In the embodiment depicted in FIG. 1, the spacerlayer is fabricated from Scandium (Sc), and the absorber layer isfabricated from Chromium (Cr). In one embodiment, the set of multilayercoatings 12 includes four hundred repeated pairs of multilayer coatings.The spatial period, P, of the set of multilayer coatings (i.e.,thickness of each repeated material pair) is 1.56 nanometers to satisfythe Bragg condition. Additional description of the multilayer opticdepicted in FIG. 1 is presented in “14.5% near-normal incidencereflectance of Cr Sc x-ray multilayer mirrors of the water window,” byEriksson, Fredrik, et al., Optics letters 28-24 (2003): 2494-2496, thecontent of which is incorporated herein by reference in its entirety.

The reflectivity of multilayer optic 10 is typically extremely sensitiveto incident angle and beam energy (i.e., wavelength). FIG. 2 is a plot20 illustrative of a simulation of the reflectivity of multilayer optic10 as a function of beam energy for an angle of incidence of fivedegrees. The simulation employs the Fresnel equations assuming ideal,flat interfaces. The optical constants associated with each material(i.e., delta and beta constants) are derived using scattering factortables from the Center For X-Ray Optics (CXRO) of the materials sciencedivision of the Lawrence Berkeley National Laboratory (accessible viaInternet at http://henke.lbl.gov/optical_constants/. As depicted in FIG.2, a maximum reflectivity of 53.1% is achieved at beam energy of 399electronvolts (3.11 nanometers), which is within the range of soft x-rayradiation (e.g., 80-3,000 eV).

Broadband, soft x-ray based metrology, requires high reflectivity atsoft x-ray wavelengths. However, lower energy radiation (e.g., EUV, UV,visible, IR) contaminates soft x-ray based measurements. Unfortunately,as illustrated in FIG. 2, the traditional multilayer optic 10 employedin soft X-ray systems exhibits high reflectivity in the extremeultraviolet (EUV) spectrum (e.g., 10 eV-80 eV). This is caused by theincreasing contrast of the refractive index between air and mirrormaterials (e.g., Cr or Sc) as photon energy decreases. As a result, softx-ray based systems employing traditional multilayer optic 10 sufferfrom EUV light contamination.

Traditionally, one or more transmissive, stand-alone optical filters areemployed in an optical path of a soft x-ray system to filter lightcontamination outside the desired soft x-ray energy band, e.g., EUV, UV,visible, IR. These transmissive, stand-alone optical filters arefabricated from an extremely thin membrane of material (e.g., 5-50nanometers thick) that spans the cross-section of the x-ray beam (e.g.,hundreds of micrometers to several millimeters across). These extremelythin membranes, which are unsupported over a relatively large distance,are costly and extremely fragile. This negatively impacts thereliability and practical utility of current soft x-ray based systems.

In summary, there is a need for x-ray based systems with improvedoptics. The improved optics should enable efficient propagation of x-rayradiation over a desired energy range and rejection of radiation outsidethe desired energy range. In particular, optics capable of propagatingbroadband, soft x-ray radiation, and rejecting wavelengths in the EUV,UV, visible, and IR portions of the spectrum are desired.

SUMMARY

Optical elements that efficiently propagate x-ray radiation over adesired energy range and reject radiation outside the desired energyrange are presented herein. By way of non-limiting example, the opticalelements described herein are implemented in any of an x-ray metrologysystem, a projection lithography system, a microscopy system, anastronomical system, a spectroscopic system, a laser illumination sourceincluding laser cavities and optics, a synchrotron illumination source,etc.

In one aspect, one or more optical elements of an x-ray based systeminclude an integrated optical filter including one or more materiallayers that absorb radiation having energy outside the desired energyband. In general, the integrated filter improves the optical performanceof an x-ray based system by suppressing reflectivity within infrared(IR), visible (vis), ultraviolet (UV), extreme ultraviolet (EUV)portions of the spectrum, or any other undesired wavelength region.

In some embodiments, a multilayer X-ray reflecting optic includes anintegrated optical filter including one or more material layers thatabsorb radiation having energy below the desired energy band. Theintegrated optical filter may include a single material layer, twomaterial layers, or more than two material layers depending on thedesired wavelength range or wavelength ranges to be absorbed.

In a further aspect, an integrated optical filter includes one or morediffusion barrier layers to prevent degradation of the integratedoptical filter by the external environment, prevent diffusion betweenthe integrated optical filter and other material layers, or both. Inmany embodiments, a thin diffusion barrier layer effectively increasesthe lifetime of a multilayer optic without affecting its opticalperformance.

In another aspect, the thicknesses of the one or more layers of anintegrated optical filter vary over the spatial area of the filter.

The foregoing is a summary and thus contains, by necessity,simplifications, generalizations and omissions of detail; consequently,those skilled in the art will appreciate that the summary isillustrative only and is not limiting in any way. Other aspects,inventive features, and advantages of the devices and/or processesdescribed herein will become apparent in the non-limiting detaileddescription set forth herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts an illustration of a cross-sectional view of a multilayeroptic 10 used in soft x-ray applications.

FIG. 2 is a plot 20 illustrative of a simulation of the reflectivity ofmultilayer optic 10 as a function of beam energy for an angle ofincidence of five degrees.

FIG. 3 depicts a multilayer X-ray reflecting optic 100 including anintegrated optical filter 101 in one embodiment.

FIG. 4 depicts a plot 110 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 100 depicted in FIG.3.

FIG. 5 depicts a plot 120 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 100 depicted in FIG. 3for different thicknesses of an integrated optical filter.

FIG. 6 depicts a multilayer X-ray reflecting optic 130 including anintegrated optical filter 131 in one embodiment.

FIG. 7 depicts a plot 140 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG.6.

FIG. 8 depicts a plot 150 illustrative of the simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 7over a contamination band from 30 eV to 130 eV.

FIG. 9 depicts a plot 160 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over a photon energy band from 30 eV to 70 eV for various thicknesses ofTe layer 133 and a constant thickness of 10.5 nanometers for SiO₂ layer132.

FIG. 10 depicts a plot 170 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over a photon energy band from 30 eV to 70 eV for various thicknesses ofSiO₂ layer 132 and a constant thickness of 14.4 nanometers for Te layer133.

FIG. 11 depicts an illustration of a table 180 summarizing the averagereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over three different contamination bands and reflectivity at a softX-ray wavelength of interest (397.9 eV) for the different combinationsof Te and SiO2 layer thicknesses illustrated in FIGS. 9 and 10.

FIG. 12 depicts a multilayer X-ray reflecting optic 190 including anintegrated optical filter 191.

FIG. 13 depicts a curved optical element 260 including an integratedoptical filter 262 disposed on the surface of the curved opticalelement.

FIG. 14 illustrates an embodiment of a RSAXS metrology tool 200 formeasuring characteristics of a specimen in at least one novel aspect.

FIG. 15 is a simplified diagram illustrative of an end view of focusingoptics including four mirror elements disposed around the beam axis, A,in a segmented toroidal configuration.

FIG. 16 depicts x-ray illumination beam incident on a wafer at aparticular orientation described by an angle of incidence, θ, and anazimuth angle, ϕ.

FIG. 17 illustrates another embodiment of a RSAXS metrology tool 300 formeasuring characteristics of a specimen in at least one novel aspect.

DETAILED DESCRIPTION

Reference will now be made in detail to background examples and someembodiments of the invention, examples of which are illustrated in theaccompanying drawings.

Optical elements that efficiently propagate x-ray radiation over adesired energy range and reject radiation outside the desired energyrange are presented herein. In addition, x-ray based metrology systemsincluding optical elements that efficiently propagate x-ray radiationover a desired energy range and reject radiation outside the desiredenergy range are also presented. In particular, broadband, soft x-raybased metrology systems employ optical elements that propagatebroadband, soft x-ray radiation and reject wavelengths in the EUV, UV,visible, IR, or any combination thereof.

In one aspect, one or more optical elements of an x-ray based systeminclude an integrated optical filter including one or more materiallayers that absorb radiation having energy below the desired energyband. In some embodiments, the integrated optical filter absorbsradiation having wavelengths above 10 nanometers (i.e., less than 123.9electronvolts). In some embodiments, the integrated optical filterabsorbs radiation having wavelengths above 13.7 nanometers (i.e., lessthan 90 electronvolts). In some embodiments, the integrated opticalfilter absorbs radiation having wavelengths above 12.4 nanometers (i.e.,less than 100 electronvolts). In some embodiments, the integratedoptical filter absorbs radiation having wavelengths above 10.3nanometers (i.e., less than 120 electronvolts). In general, theintegrated filter improves the optical performance of an x-ray basedsystem by suppressing reflectivity within infrared (IR), visible (vis),ultraviolet (UV), extreme ultraviolet (EUV) portions of the spectrum, orany other undesired wavelength region.

By way of non-limiting example, one or more optical elements of an x-raymetrology system, a projection lithography system, a microscopy system,an astronomical system, a spectroscopic system, a laser illuminationsource including laser cavities and optics, a synchrotron illuminationsource, etc., include an integrated optical filter to absorb undesiredradiation.

In some embodiments, a multilayer X-ray reflecting optic includes anintegrated optical filter including one or more material layers thatabsorb radiation having energy below the desired energy band. Theintegrated optical filter may include a single material layer, twomaterial layers, or more than two material layers depending on thedesired wavelength range or wavelength ranges to be absorbed. Typically,the thickness of each layer is selected for optimal n phase-matching,i.e., 180 degree phase matching, to maximize extinction at over therange of photon energy to be suppressed. The thickness of each materiallayer usually lies in a range between one and one hundred nanometers.

For an integrated optical filter including a single material layerdisposed on top of a set of repeated pairs of reflective multilayercoatings, an optical refractive index, n_(filter), and thickness,t_(filter), of the single material layer may be approximated byequations (1) and (2), where n_(top) is the index of refraction of thetop layer of the underlying set of repeated pairs of reflectivemultilayer coatings, θ, is the angle of incidence of the incident beam,and λ is the wavelength of the incident beam.

$\begin{matrix}n_{{filter} = \sqrt{n_{top}}} & (1) \\t_{{filter} = \frac{{\lambda\cos}{(\theta)}}{4}} & (2)\end{matrix}$

In practice, a selection of material and deposition thickness for anintegrated optical filter is guided by performing a series of thicknessoptimizations for a variety of materials guided by equations (1) and(2). For example, the complexity of a multilayer structure may shift theoptimal thickness to achieve an optimal extinction valley away from thethickness estimate made by Equation (2). In addition, it is alsochallenging to identify a material that can be effectively deposited ina thin layer that perfectly matches the requirement of Equation (1).Thus, in practice, equation (1) helps to narrow the list of candidatematerials, and equation (2) offers a good starting point for thicknessoptimization.

In addition, it is rare that out of band contamination (e.g., EUVcontamination) lies within a sharp wavelength band. Thus, the selectionand thickness optimization of a single material layer will often involvecompromises regarding out of band absorption. If these compromisesrender the single layer filter ineffective, one or more additionallayers of different material(s) should be considered.

FIG. 3 depicts a multilayer X-ray reflecting optic 100 including anintegrated optical filter 101. Like numbered elements depicted in FIG. 3are analogous to those described with reference to FIG. 1. As depictedin FIG. 3, integrated optical filter 101 includes a material layerexplicitly fabricated on top of the set of repeated pairs of multilayercoatings 12. In one example, integrated optical filter 101 is a silicondioxide (SiO₂) layer deposited on top of chromium absorber layer 14.

FIG. 4 depicts a plot 110 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 100 depicted in FIG.3. Plotline 111 depicts the reflectivity of multilayer x-ray reflectingoptic 100 as a function of beam energy without integrated optical filter101. Plotline 112 depicts the reflectivity of multilayer x-rayreflecting optic 100 as a function of beam energy with an integratedoptical filter 101 having a single layer of SiO₂ with a thickness of 10nanometers. As illustrated in FIG. 4, the integrated optical filter 101eliminates 99.3% of the radiation at 41.6 eV, and only absorbs 5.4% ofthe radiation at the wavelength of interest (397.9 eV). Over acontamination band from 30 eV to 130 eV, the integrated optical filterdecreases average reflectivity by a factor of eight compared to theunfiltered scenario.

FIG. 5 depicts a plot 120 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 100 depicted in FIG. 3for different thicknesses of an integrated optical filter. Plotline 121depicts a simulation of reflectivity of multilayer x-ray reflectingoptic 100 with an integrated optical filter 101 having a single layer ofSiO₂ with a thickness of 8 nanometers. Plotline 122 depicts a simulationof reflectivity of multilayer x-ray reflecting optic 100 with anintegrated optical filter 101 having a single layer of SiO₂ with athickness of 10 nanometers. Plotline 123 depicts a simulation ofreflectivity of multilayer x-ray reflecting optic 100 with an integratedoptical filter 101 having a single layer of SiO₂ with a thickness of 12nanometers. As the thickness changes from 8 nanometers to 12 nanometers,the first extinction valley shifts from 47.0 eV (26.4 nanometers) to39.1 eV (31.7 nanometers). In addition, the second extinction valleyshifts from 115.1 eV (10.8 nanometers) to 83.9 eV (14.8 nanometers). Inthis manner, the stopband of integrated optical filter 101 is tuned byadjusting its thickness.

In some embodiments, the bandwidth of a single-layer integrated opticalfilter is too narrow to sufficiently suppress the range of contaminationwavelengths of interest. In these embodiments, an integrated opticalfilter having two layers, or more than two layers is employed. In someembodiments, a multiple layer integrated optical filter may absorb lessradiation at the desired wavelengths of x-ray system operation.

In general, the order of layers of the multiple layer integrated opticalfilter may be arbitrary. However, in preferred embodiments, the materiallayer of the multiple layer integrated optical filter that best matchesthe refractive index of air (e.g., smaller delta and beta) is disposedon top of the multiple layer stack to allow more radiation through theair/layer interface.

FIG. 6 depicts a multilayer X-ray reflecting optic 130 including anintegrated optical filter 131. Like numbered elements depicted in FIG. 6are analogous to those described with reference to FIG. 1. As depictedin FIG. 6, integrated optical filter 131 includes two material layers132 and 133 explicitly fabricated on top of the set of repeated pairs ofmultilayer coatings 12. In one example, layer 132 is a silicon dioxide(SiO₂) layer deposited on top of chromium absorber layer 14, and layer133 is a Tellurium (Te) layer deposited on top of SiO₂ layer 132. Telayer 133 is deposited on top because it has significantly smaller deltaand beta values compared to SiO₂ below 60 eV. Integrated optical filter131 is preferred in x-ray systems where contamination is mainly focusedwithin a range of photon energy from 30 eV to 60 eV.

FIG. 7 depicts a plot 140 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG.6. Plotline 141 depicts the reflectivity of multilayer x-ray reflectingoptic 130 as a function of photon energy without integrated opticalfilter 131. Plotline 142 depicts the reflectivity of multilayer x-rayreflecting optic 100 as a function of photon energy with an integratedoptical filter 131 having a layer 132 of SiO₂ with a thickness of 10.5nanometers and a layer 133 of Te with a thickness of 14.4 nanometers. Asillustrated in FIG. 7, the integrated optical filter 131 has a muchwider stopband compared to integrated optical filter 101 illustrated inFIG. 4. Integrated optical filter 131 includes three extinction valleysbelow 60 eV. In addition, integrated optical filter 131 only absorbs13.8% of the radiation at the wavelength of interest (397.9 eV) comparedto the unfiltered scenario.

FIG. 8 depicts a plot 150 illustrative of the simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 7over a contamination band from 30 eV to 130 eV. As depicted in FIG. 8,peak extinction occurs at 41.2 eV, where 98.9% of photon energy isabsorbed. Integrated optical filter 130 decreases average reflectivityby a factor of seventeen compared to the unfiltered scenario over acontamination band of interest from 30 eV to 60 eV.

FIG. 9 depicts a plot 160 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over a photon energy band from 30 eV to 70 eV for various thicknesses ofTe layer 133 and a constant thickness of 10.5 nanometers for SiO₂ layer132. Plotline 161 depicts reflectivity of multilayer x-ray reflectingoptic 130 with Te layer 133 having a thickness of 13.4 nanometers.Plotline 162 depicts reflectivity of multilayer x-ray reflecting optic130 with Te layer 133 having a thickness of 14.4 nanometers. Plotline163 depicts reflectivity of multilayer x-ray reflecting optic 130 withTe layer 133 having a thickness of 15.4 nanometers.

FIG. 10 depicts a plot 170 illustrative of a simulation of thereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over a photon energy band from 30 eV to 70 eV for various thicknesses ofSiO₂ layer 132 and a constant thickness of 14.4 nanometers for Te layer133. Plotline 171 depicts reflectivity of multilayer x-ray reflectingoptic 130 with SiO₂ layer 132 having a thickness of 9.5 nanometers.Plotline 172 depicts reflectivity of multilayer x-ray reflecting optic130 with SiO₂ layer 132 having a thickness of 10.5 nanometers. Plotline173 depicts reflectivity of multilayer x-ray reflecting optic 130 withSiO₂ layer 132 having a thickness of 11.5 nanometers. As illustrated inFIGS. 9 and 10, changing the thickness of Te layer 133 affects thelocation of the 1^(st) extinction valley 164 and 3^(rd) extinctionvalley 166, whereas changing the thickness of SiO₂ layer 132 shifts thelocation of the 2nd extinction valley 165.

FIG. 11 depicts an illustration of a table 180 summarizing the averagereflectivity of multilayer x-ray reflecting optic 130 depicted in FIG. 6over three different contamination bands and reflectivity at a softX-ray wavelength of interest (397.9 eV) for the different combinationsof Te and SiO2 layer thicknesses illustrated in FIGS. 9 and 10. In oneexample, the output spectrum of the illumination source of an x-raysystem dictates the contamination waveband where maximum suppression isdesired. Table 180 is used to select the best combination of filmthicknesses to suppress photon energy within the identifiedcontamination waveband.

In some embodiments, an X-ray optical element is designed to supportmultiple wavelengths of interest. In some of these embodiments, theintegrated optical filter includes a single layer or multi-layercombination optimized over all of the wavelengths of interest. In someother embodiments, the integrated optical filter includes multiplesingle layers or multiple multi-layer combinations each optimized over adifferent portion of all of the wavelengths of interest, and eachoptimized to suppress at least one contamination band.

In a further aspect, an integrated optical filter includes one or morediffusion barrier layers to prevent degradation of the integratedoptical filter by the external environment, prevent diffusion betweenthe integrated optical filter and other material layers, or both. Inmany embodiments, a thin diffusion barrier layer effectively increasesthe lifetime of a multilayer optic without affecting its opticalperformance.

FIG. 12 depicts a multilayer X-ray reflecting optic 190 including anintegrated optical filter 191. Like numbered elements depicted in FIG.12 are analogous to those described with reference to FIGS. 1 and 6. Asdepicted in FIG. 12, integrated optical filter 191 includes a diffusionbarrier layer 192 deposited on top of chromium absorber layer 14 andunder SiO₂ layer 132. In this embodiment, diffusion barrier layer 192prevents oxygen and water from degrading the set of repeated pairs ofmultilayer coatings 12, and also protects the set of repeated pairs ofmultilayer coatings 12 from diffusing into SiO₂ layer 132 and Te layer133. In another embodiment, diffusion barrier layer 192 is deposited ontop of SiO₂ layer 132 and under Te layer 133. In yet another embodiment,diffusion barrier layer 192 is deposited on top of Te layer 133. Inthese embodiments, diffusion barrier layer 192 prevents oxygen and waterfrom degrading the set of repeated pairs of multilayer coatings 12. Ingeneral, one or more diffusion barrier layers may be deposited anywherein the stack of an integrated optical filter.

In general, an integrated optical filter may be located anywhere in thestack of a multiple layered optical element. In some embodiments, anintegrated optical filter is located on top of the layer stack of amultilayer X-ray reflecting optic as illustrated in FIGS. 3, 6, and 12.In some other embodiments, an integrated optical filter is located belowa set of repeated pairs of multilayer coatings (e.g., between the set ofrepeated pairs of multilayer coatings 12 and substrate 11) in the layerstack of a multilayer X-ray reflecting optic.

In some embodiments, one or more of the layers of an integrated opticalfilter are fabricated from chemically inert materials to improvestability and lifetime of the optical element. In these embodiments, theintegrated optical filter is located on top of the layer stack of amultilayer X-ray reflecting optic, separating the set of repeated pairsof multilayer coatings from the external environment. In this manner,the integrated optical filter also acts as a protective layer thatprevents contamination of the repeated pairs of multilayer coatings bythe surrounding environment.

By way of non-limiting example, materials that are both chemically inertand suitable candidates for one or more layers of an integrated opticalfilter include pure elements (e.g. tellurium, carbon, magnesium) andcompounds (e.g. boron carbide (B₄C), silicon nitride (Si₃N₄), siliconoxide (SiO₂)). These materials can be deposited in thin layers on top ofmultilayer reflective coatings by various deposition techniques, such assputtering. In addition, the deposition of these material layers can beperformed directly by the fabrication tool employed to fabricate themultilayer reflective coatings (e.g., magnetron sputtering tool).

In another aspect, the thicknesses of the one or more layers of anintegrated optical filter vary over the spatial area of the filter. Insome embodiments, an x-ray optical element is curved and the one or morelayers of an integrated optical filter have gradient thickness thattracks the incidence angle along the curved optic such that the incidentlight has the same path length through the absorbing material regardlessof location of incidence along the curved optic. In this manner, thesuppression efficiency of the integrated optical element is uniformalong the entire optical surface.

FIG. 13 depicts a curved optical element 260 including an integratedoptical filter 262 disposed on the surface of the curved opticalelement. Incident light 266 from a source 264 reflects from curvedoptical element 260. The reflected light 267 is focused to a focal area265 by curved optical element 260. Incident light 266 is incident on thesurface of curved optical element 260 over a relatively large area. Inother words, different portions of the incident beam reflect fromdifferent locations on the surface of curved optical element 260 havingsignificantly different curvature. For example, light incident at point268 is incident to the surface of curved optical element 260 at anangle, α₁, and light incident at point 269 is incident at a differentangle, α₂. As depicted in FIG. 13, the light incident at point 268traverses a path length, L₁, through integrated optical filter 262. Thepath length, L₁, is related to the angle of incidence, a, and thethickness, T₁, of the integrated optical element 262 at point 268 asdescribed by equation (3).

$\begin{matrix}{L_{1} = \frac{T_{1}}{\sin\left( \alpha_{1} \right)}} & (3)\end{matrix}$Also, as depicted in FIG. 13, the light incident at point 269 traversesa path length, L₂, through integrated optical filter 262. The pathlength, L₂, is related to the angle of incidence, α₂, and the thickness,T₂, of the integrated optical element 262 at point 269 as described byequation (4).

$\begin{matrix}{L_{2} = \frac{T_{2}}{\sin\;\left( \alpha_{2} \right)}} & (4)\end{matrix}$In the depicted embodiment, the thickness, T₁, of integrated filterelement 262 at point 268 and the thickness of integrated filter element262 at point 269 are selected to maintain the same path length throughintegrated optical filter 262 at both locations in accordance withequation (5).

$\begin{matrix}{\frac{T_{1}}{\sin\;\left( \alpha_{1} \right)} = \frac{T_{2}}{\sin\;\left( \alpha_{2} \right)}} & (5)\end{matrix}$

X-ray based metrology systems are employed to measure structural andmaterial characteristics (e.g., material composition, dimensionalcharacteristics of structures and films, etc.) of semiconductorstructures associated with different semiconductor fabrication processesbased on x-ray illumination.

In some embodiments, an x-ray based metrology system performsmeasurements of semiconductor structures based on high-brightness,polychromatic reflective small angle x-ray scatterometry (RSAXS).Further description is provided in U.S. Patent Publication No.2019/0017946 by Wack et al., the content of which is incorporated hereinby reference in its entirety.

RSAXS measurements of a semiconductor wafer are performed over a rangeof wavelengths, angles of incidence, and azimuth angles with a smallbeam spot size (e.g., less than 50 micrometers across the effectiveillumination spot). In one aspect, RSAXS measurements are performed withx-ray radiation in the soft x-ray (SXR) region (i.e., 80-3000 eV) atgrazing angles of incidence in the range of 5-20 degrees. Grazing anglesfor a particular measurement application are selected to achieve adesired penetration into the structure under measurement and maximizemeasurement information content with a small beam spot size (e.g., lessthan 50 micrometers).

FIG. 14 illustrates an embodiment of a RSAXS metrology tool 200 formeasuring characteristics of a specimen. As shown in FIG. 14, the system100 may be used to perform RSAXS measurements over a measurement area202 of a specimen 201 illuminated by an incident illumination beam spot.

In the depicted embodiment, metrology tool 200 includes an x-rayillumination source 210, focusing optics 211, beam divergence controlslit 212, and slit 213. The x-ray illumination source 210 is configuredto generate SXR radiation suitable for RSAXS measurements. X-rayillumination source 210 is a polychromatic, high-brightness, largeetendue source. In some embodiments, the x-ray illumination source 210is configured to generate x-ray radiation in a range between 80-3000electron-volts. In general, any suitable high-brightness x-rayillumination source capable of generating high brightness SXR at fluxlevels sufficient to enable high-throughput, inline metrology may becontemplated to supply x-ray illumination for RSAXS measurements.

By way of non-limiting example, any of a particle accelerator source, aliquid anode source, a rotating anode source, a stationary, solid anodesource, a microfocus source, a microfocus rotating anode source, aplasma based source, and an inverse Compton source may be employed asx-ray illumination source 210.

Exemplary x-ray sources include electron beam sources configured tobombard solid or liquid targets to stimulate x-ray radiation. Methodsand systems for generating high brightness, liquid metal x-rayillumination are described in U.S. Pat. No. 7,929,667, issued on Apr.19, 2011, to KLA-Tencor Corp., the entirety of which is incorporatedherein by reference.

In some embodiments, an x-ray source includes a tunable monochromatorthat enables the x-ray source to deliver x-ray radiation at different,selectable wavelengths. In some embodiments, one or more x-ray sourcesare employed to ensure that the x-ray source supplies light atwavelengths that allow sufficient penetration into the specimen undermeasurement.

In some embodiments, illumination source 210 is a high harmonicgeneration (HHG) x-ray source. In some other embodiments, illuminationsource 210 is a wiggler/undulator synchrotron radiation source (SRS). Anexemplary wiggler/undulator SRS is described in U.S. Pat. Nos. 8,941,336and 8,749,179, the contents of which are incorporated herein byreference in their entireties.

In some other embodiments, illumination source 210 is a laser producedplasma (LPP) light source. In some of these embodiments the LPP lightsource includes any of Xenon, Krypton, Argon, Neon, and Nitrogenemitting materials. In general, the selection of a suitable LPP targetmaterial is optimized for brightness in resonant S×R regions. Forexample, plasma emitted by Krypton provides high brightness at theSilicon K-edge. In another example, plasma emitted by Xenon provideshigh brightness throughout the entire S×R region of (80-3000 eV). Assuch, Xenon is a preferred choice of emitting material when broadbandSXR illumination is desired.

LPP target material selection may also be optimized for reliable andlong lifetime light source operation. Noble gas target materials such asXenon, Krypton, and Argon are inert and can be reused in a closed loopoperation with minimum or no decontamination processing. An exemplarySXR illumination source is described in U.S. Patent Publication No.2019/0215940, the content of which is incorporated herein by referencein its entirety.

In a further aspect, the wavelengths emitted by the illumination source(e.g., illumination source 210) are selectable. In some embodiments,illumination source 210 is a LPP light source controlled by computingsystem 230 to maximize flux in one or more selected spectral regions.Laser peak intensity at the target material controls the plasmatemperature and thus the spectral region of emitted radiation. Laserpeak intensity is varied by adjusting pulse energy, pulse width, orboth. In one example, a 100 picosecond pulse width is suitable forgenerating SXR radiation. As depicted in FIG. 14, computing system 230communicates command signals 236 to illumination source 210 that causeillumination source 210 to adjust the spectral range of wavelengthsemitted from illumination source 210. In one example, illuminationsource 210 is a LPP light source, and the LPP light source adjusts anyof a pulse duration, pulse frequency, and target material composition torealize a desired spectral range of wavelengths emitted from the LPPlight source.

X-ray illumination source 210 produces x-ray emission over a source areahaving finite lateral dimensions (i.e., non-zero dimensions orthogonalto the beam axis. In one aspect, the source area of illumination source210 is characterized by a lateral dimension of less than 20 micrometers.In some embodiments, the source area is characterized by a lateraldimension of 10 micrometers or less. Small source size enablesillumination of a small target area on the specimen with highbrightness, thus improving measurement precision, accuracy, andthroughput.

In general, x-ray optics shape and direct x-ray radiation to specimen201. In some examples, the x-ray optics collimate or focus the x-raybeam onto measurement area 202 of specimen 201 to less than 1milliradian divergence using multilayer x-ray optics. In someembodiments, the x-ray optics include one or more x-ray collimatingmirrors, x-ray apertures, x-ray beam stops, refractive x-ray optics,diffractive optics such as zone plates, Schwarzschild optics,Kirkpatrick-Baez optics, Montel optics, Wolter optics, specular x-rayoptics such as ellipsoidal mirrors, polycapillary optics such as hollowcapillary x-ray waveguides, multilayer optics or systems, or anycombination thereof. Further details are described in U.S. PatentPublication No. 2015/0110249, the content of which is incorporatedherein by reference it its entirety.

As depicted in FIG. 14, focusing optics 211 focuses source radiationonto a metrology target located on specimen 201. The finite lateralsource dimension results in finite spot size 202 on the target definedby the rays 216 coming from the edges of the source and any beam shapingprovided by beam slits 212 and 213. In some embodiments, a multilayerx-ray optical element of an x-ray based metrology system, such asfocusing optical element 211 of RSAXS system 200, includes an integratedoptical filter as described herein.

In some embodiments, focusing optics 211 includes elliptically shapedfocusing optical elements. In the embodiment depicted in FIG. 14, themagnification of focusing optics 211 at the center of the ellipse isapproximately one. As a result, the illumination spot size projectedonto the surface of specimen 201 is approximately the same size as theillumination source, adjusted for beam spread due to the nominal grazingincidence angle (e.g., 5-20 degrees).

In a further aspect, focusing optics 211 collect source emission andselect one or more discrete wavelengths or spectral bands, and focus theselected light onto specimen 201 at grazing angles of incidence in therange 5-20 degrees.

The nominal grazing incidence angle is selected to achieve a desiredpenetration of the metrology target to maximize signal informationcontent while remaining within metrology target boundaries. The criticalangle of hard x-rays is very small, but the critical angle of softx-rays is significantly larger. As a result of this additionalmeasurement flexibility RSAXS measurements probe more deeply into thestructure with less sensitivity to the precise value of the grazingincidence angle.

In some embodiments, focusing optics 211 include graded multi-layersthat select desired wavelengths or ranges of wavelengths for projectiononto specimen 201. In some examples, focusing optics 211 includes agraded multi-layer structure (e.g., layers or coatings) including anintegrated optical filter that selects one wavelength and projects theselected wavelength onto specimen 201 over a range of angles ofincidence. In some examples, focusing optics 211 includes a gradedmulti-layer structure including an integrated optical filter thatselects a range of wavelengths and projects the selected wavelengthsonto specimen 201 over one angle of incidence. In some examples,focusing optics 211 includes a graded multi-layer structure including anintegrated optical filter that selects a range of wavelengths andprojects the selected wavelengths onto specimen 201 over a range ofangles of incidence.

Graded multi-layered optics including an integrated optical filter arepreferred to minimize loss of light that occurs when single layergrating structures are too deep. In general, multi-layer optics selectreflected wavelengths. The spectral bandwidth of the selectedwavelengths optimizes flux provided to specimen 201, information contentin the measured diffracted orders, and prevents degradation of signalthrough angular dispersion and diffraction peak overlap at the detector.In addition, graded multi-layer optics are employed to controldivergence. Angular divergence at each wavelength is optimized for fluxand minimal spatial overlap at the detector.

In some examples, graded multi-layer optics including an integratedoptical filter select wavelengths to enhance contrast and informationcontent of diffraction signals from specific material interfaces orstructural dimensions, and suppress contamination wavelengths, such aswavelengths greater than 10 nanometers. For example, the selectedwavelengths may be chosen to span element-specific resonance regions(e.g., Silicon K-edge, Nitrogen, Oxygen K-edge, etc.). In addition, inthese examples, the illumination source may also be tuned to maximizeflux in the selected spectral region (e.g., HHG spectral tuning, LPPlaser tuning, etc.)

In some other examples, little to no prior structural information isavailable at the time of measurement. In these examples, multiplewavelengths (e.g., 3-4) are selected to enable measurement ofdiffraction patterns across the absorption edge. The measured signalsenable model-free measurement of structural properties with no priorinformation except the elemental composition of the structures undermeasurement using, for example, multiple wavelength anomalousdiffraction techniques. After estimating structural properties based onmodel-free measurements, parameter estimates may be further refinedusing model-based measurement techniques.

In some examples, the anomalous scattering factors (i.e., scatteringproperties) of the metrology target under measurement are not knownapriori. In these examples, film multilayer reflectivity is measured atmultiple resonant wavelengths. Angular excursions of Bragg peaks providesufficient information to extract the anomalous scattering factors.

In some examples, non-resonant x-ray reflectivity measurements provideindependent estimates of multilayer period and interface roughnessparameters, which improve the fitting of model-based measurements. Insome embodiments, a combined metrology tool includes a multiplewavelength SXR diffraction subsystem as described herein and an x-rayreflectometry subsystem to improve measurement throughput. In oneembodiment, the multiple wavelength SXR diffraction subsystem and thex-ray reflectometry subsystem employ orthogonal planes of incidence thatenable simultaneous measurements or sequential measurements withouthaving to move the specimen under measurement or either of the opticalmeasurement subsystems. In some embodiments, wafer rotation, detectorrotation, or both, may be employed to extend the range of angles ofincidence if the AOI range provided by the SXR multilayer mirrors is toosmall for x-ray reflectometry.

In some embodiments, focusing optics 211 include a plurality ofreflective optical elements each having an elliptical surface shape.Each reflective optical element includes a substrate and a multi-layercoating and integrated optical filter tuned to reflect a differentwavelength or range of wavelengths and suppress a different wavelengthor range of wavelengths.

In some embodiments, focusing optics 211 focus light at multiplewavelengths, azimuths and AOI on the same metrology target area. FIG. 15depicts an end view (i.e., along the beam axis) of focusing optics 250including four mirror elements 250A-250D disposed around the beam axis,A, in a segmented toroidal configuration. Each mirror element includes amulti-layer coating tuned to reflect a different wavelength or range ofwavelengths and an integrated optical filter tuned to suppress adifferent wavelength or range of wavelengths. In some embodiments, eachmirror element 250A-D includes a uniform multilayer design (i.e., thesurface of a particular mirror element reflects the same wavelength orrange of wavelengths over the entire mirror surface area of thatparticular mirror element). In some other embodiments, each mirrorelement includes a non-uniform multilayer design (i.e., the wavelengthor range of wavelengths reflected by the mirror element depends on thelocation of incidence on the mirror surface). In some of theseembodiments, each mirror element is elliptical in shape and projectsillumination light to specimen 201 over a range of angles of incidence.In addition, because the mirror elements are arranged in a toroidalconfiguration, the mirror elements project illumination light tospecimen 201 over a range of azimuth angles. Although, FIG. 15 depictsfour mirror elements, in general, focusing optics may include any numberof mirror elements arranged to focus light at multiple wavelengths,azimuths and AOI on the same metrology target area. In some otherembodiments, focusing optics includes a number of mirror elements nestedin the plane of incidence (i.e., a nested Wolter configuration).

In a further aspect, the ranges of wavelengths, AOI, Azimuth, or anycombination thereof, projected onto the same metrology area, areadjusted by actively positioning one or more mirror elements of thefocusing optics. As depicted in FIG. 14, computing system 230communicates command signals to actuator system 215 that causes actuatorsystem 215 to adjust the position, alignment, or both, of one or more ofthe optical elements of focusing optics 211 to achieve the desiredranges of wavelengths, AOI, Azimuth, or any combination thereof,projected onto specimen 201.

In general, the angle of incidence is selected for each wavelength tooptimize penetration and absorption of the illumination light by themetrology target under measurement. In many examples, multiple layerstructures are measured and angle of incidence is selected to maximizesignal information associated with the desired layers of interest. Inthe example of overlay metrology, the wavelength(s) and angle(s) ofincidence are selected to maximize signal information resulting frominterference between scattering from the previous layer and the currentlayer. In addition, azimuth angle is also selected to optimize signalinformation content. In addition, azimuth angle is selected to ensureangular separation of diffraction peaks at the detector.

In a further aspect, an RSAX metrology system (e.g., metrology tool 200)includes one or more beam slits or apertures to shape the illuminationbeam 214 incident on specimen 201 and selectively block a portion ofillumination light that would otherwise illuminate a metrology targetunder measurement. One or more beam slits define the beam size and shapesuch that the x-ray illumination spot fits within the area of themetrology target under measurement. In addition, one or more beam slitsdefine illumination beam divergence to minimize overlap of diffractionorders on the detector.

In another further aspect, an RSAX metrology system (e.g., metrologytool 200) includes one or more beam slits or apertures to select a setof illumination wavelengths that simultaneously illuminate a metrologytarget under measurement. In some embodiments, illumination includingmultiple wavelengths is simultaneously incident on a metrology targetunder measurement. In these embodiments, one or more slits areconfigured to pass illumination including multiple illuminationwavelengths. In general, simultaneous illumination of a metrology targetunder measurement is preferred to increase signal information andthroughput. However, in practice, overlap of diffraction orders at thedetector limits the range of illumination wavelengths. In someembodiments, one or more slits are configured to sequentially passdifferent illumination wavelengths. In some examples, sequentialillumination at larger angular divergence provides higher throughputbecause the signal to noise ratio for sequential illumination may behigher compared to simultaneous illumination when beam divergence islarger. When measurements are performed sequentially the problem ofoverlap of diffraction orders is not an issue. This increasesmeasurement flexibility and improves signal to noise ratio.

FIG. 14 depicts a beam divergence control slit 212 located in the beampath between focusing optics 211 and beam shaping slit 213. Beamdivergence control slit 212 limits the divergence of the illuminationprovided to the specimen under measurement. Beam shaping slit 213 islocated in the beam path between beam divergence control slit 212 andspecimen 201. Beam shaping slit 213 further shapes the incident beam 214and selects the illumination wavelength(s) of incident beam 214. Beamshaping slit 213 is located in the beam path immediately before specimen201. In one aspect, the slits of beam shaping slit 213 are located inclose proximity to specimen 201 to minimize the enlargement of theincident beam spot size due to beam divergence defined by finite sourcesize.

In some embodiments, beam shaping slit 213 includes multiple,independently actuated beam shaping slits. In one embodiment, beamshaping slit 213 includes four independently actuated beam shapingslits. These four beams shaping slits effectively block a portion of theincoming beam and generate an illumination beam 214 having a box shapedillumination cross-section.

Slits of beam shaping slit 213 are constructed from materials thatminimize scattering and effectively block incident radiation. Exemplarymaterials include single crystal materials such as Germanium, GalliumArsenide, Indium Phosphide, etc. Typically, the slit material is cleavedalong a crystallographic direction, rather than sawn, to minimizescattering across structural boundaries. In addition, the slit isoriented with respect to the incoming beam such that the interactionbetween the incoming radiation and the internal structure of the slitmaterial produces a minimum amount of scattering. The crystals areattached to each slit holder made of high density material (e.g.,tungsten) for complete blocking of the x-ray beam on one side of theslit.

X-ray detector 219 collects x-ray radiation 218 scattered from specimen201 and generates output signals 235 indicative of properties ofspecimen 201 that are sensitive to the incident x-ray radiation inaccordance with a RSAXS measurement modality. In some embodiments,scattered x-rays 218 are collected by x-ray detector 219 while specimenpositioning system 240 locates and orients specimen 101 to produceangularly resolved scattered x-rays.

In some embodiments, a RSAXS system includes one or more photon countingdetectors with high dynamic range (e.g., greater than 10⁵). In someembodiments, a single photon counting detector detects the position andnumber of detected photons.

In some embodiments, the x-ray detector resolves one or more x-rayphoton energies and produces signals for each x-ray energy componentindicative of properties of the specimen. In some embodiments, the x-raydetector 219 includes any of a CCD array, a microchannel plate, aphotodiode array, a microstrip proportional counter, a gas filledproportional counter, a scintillator, or a fluorescent material.

In this manner the X-ray photon interactions within the detector arediscriminated by energy in addition to pixel location and number ofcounts. In some embodiments, the X-ray photon interactions arediscriminated by comparing the energy of the X-ray photon interactionwith a predetermined upper threshold value and a predetermined lowerthreshold value. In one embodiment, this information is communicated tocomputing system 230 via output signals 235 for further processing andstorage.

Diffraction patterns resulting from simultaneous illumination of aperiodic target with multiple illumination wavelengths are separated atthe detector plane due to angular dispersion in diffraction. In theseembodiments, integrating detectors are employed. The diffractionpatterns are measured using area detectors, e.g., vacuum-compatiblebackside CCD or hybrid pixel array detectors. Angular sampling isoptimized for Bragg peak integration. If pixel level model fitting isemployed, angular sampling is optimized for signal information content.Sampling rates are selected to prevent saturation of zero order signals.

In a further aspect, a RSAXS system is employed to determine propertiesof a specimen (e.g., structural parameter values) based on one or morediffraction orders of scattered light. As depicted in FIG. 14, metrologytool 100 includes a computing system 130 employed to acquire signals 235generated by detector 219 and determine properties of the specimen basedat least in part on the acquired signals.

In some examples, metrology based on RSAXS involves determining thedimensions of the sample by the inverse solution of a pre-determinedmeasurement model with the measured data. The measurement model includesa few (on the order of ten) adjustable parameters and is representativeof the geometry and optical properties of the specimen and the opticalproperties of the measurement system. The method of inverse solveincludes, but is not limited to, model based regression, tomography,machine learning, or any combination thereof. In this manner, targetprofile parameters are estimated by solving for values of aparameterized measurement model that minimize errors between themeasured scattered x-ray intensities and modeled results.

It is desirable to perform measurements at large ranges of wavelength,angle of incidence and azimuth angle to increase the precision andaccuracy of measured parameter values. This approach reducescorrelations among parameters by extending the number and diversity ofdata sets available for analysis.

Measurements of the intensity of diffracted radiation as a function ofillumination wavelength and x-ray incidence angle relative to the wafersurface normal are collected. Information contained in the multiplediffraction orders is typically unique between each model parameterunder consideration. Thus, x-ray scattering yields estimation resultsfor values of parameters of interest with small errors and reducedparameter correlation.

Each orientation of the illuminating x-ray beam 214 relative to thesurface normal of a semiconductor wafer 201 is described by any twoangular rotations of wafer 201 with respect to the x-ray illuminationbeam 214, or vice-versa. In one example, the orientation can bedescribed with respect to a coordinate system fixed to the wafer. FIG.16 depicts x-ray illumination beam 214 incident on wafer 201 at aparticular orientation described by an angle of incidence, θ, and anazimuth angle, ϕ. Coordinate frame XYZ is fixed to the metrology system(e.g., illumination beam 216) and coordinate frame X′Y′Z′ is fixed towafer 201. The Y axis is aligned in plane with the surface of wafer 201.X and Z are not aligned with the surface of wafer 201. Z′ is alignedwith an axis normal to the surface of wafer 201, and X′ and Y′ are in aplane aligned with the surface of wafer 201. As depicted in FIG. 16,x-ray illumination beam 214 is aligned with the Z-axis and thus lieswithin the XZ plane. Angle of incidence, θ, describes the orientation ofthe x-ray illumination beam 214 with respect to the surface normal ofthe wafer in the XZ plane. Furthermore, azimuth angle, ϕ, describes theorientation of the XZ plane with respect to the X′Z′ plane. Together, θand ϕ, uniquely define the orientation of the x-ray illumination beam214 with respect to the surface of wafer 201.

In one aspect, metrology tool 100 includes a wafer chuck 203 thatfixedly supports wafer 201 and is coupled to specimen positioning system240. Specimen positioning system 240 configured to actively positionspecimen 201 in six degrees of freedom with respect to illumination beam214. In one example, computing system 230 communicates command signals(not shown) to specimen positioning system 240 that indicate the desiredposition of specimen 201. In response, specimen positioning system 240generates command signals to the various actuators of specimenpositioning system 240 to achieve the desired positioning of specimen201.

In a further aspect, the focusing optics of an RSAXS system projects animage of the illumination source onto the specimen under measurementwith a demagnification of at least five (i.e., magnification factor of0.2 or less). An RSAXS system as described herein employs a SXRillumination source having a source area characterized by a lateraldimension of 20 micrometers or less (i.e., source size is 20 micrometersor smaller). In some embodiments, focusing optics are employed with ademagnification factor of at least five (i.e., project an image of thesource onto the wafer that is five times smaller than the source size)to project illumination onto a specimen with an incident illuminationspot size of four micrometers or less.

FIG. 17 illustrates an embodiment of a RSAXS metrology tool 300 inanother embodiment. As illustrated in FIG. 17, the system 300 may beused to perform RSAXS measurements over a measurement area 202 havingdimensions less than 1-2 micrometers. Like numbered elements depicted inFIG. 17 are analogous to those described with reference to FIG. 14. Asdepicted in FIG. 17, focusing optics 211 are elliptical opticalelements. However, focusing optics 211 are arranged with respect toillumination source 210 and specimen 201 such that the distance, A,between illumination source 210 and focusing optics 211 is significantlygreater than the distance, B, between focusing optics 211 and specimen201. In some embodiments, the ratio of A/B is at least five. In someembodiments, the ratio of A/B is at least ten. This results in ademagnification of the illumination source onto specimen 201 by a factorof A/B. In one embodiment, the size of illumination source 210 isapproximately 10 micrometers and focusing optics 211 are arranged suchthat A/B is ten. In this embodiment, the illumination spot sizeprojected onto specimen 201 is approximately 1 micrometer.

In some embodiments, illumination source 210 is an LPP light sourcehaving a source size of 10 micrometers, or less, and focusing optics 211have a demagnification factor of approximately 10. This enables RSAXSmetrology tool 300 to focus illumination light onto a metrology targethaving dimensions of 1-2 micrometers. By focusing incident illuminationlight to an illumination spot size of 1-2 micrometers, RSAXS metrologytool 300 enables the measurement of critical dimension targets andoverlay targets located in-die, rather than relying on larger metrologytargets located in the wafer scribe line areas.

The ability to measure targets having dimensions of 1-2 micrometersreduces the wafer area committed to specialized metrology targets. Inaddition, the ability to measure targets having dimensions of 1-2micrometers enables the direct measurement of device structures, ratherthan specialized metrology targets. Measuring device structures directlyeliminates target-to-device bias. This significantly improvesmeasurement quality. In addition, measurements of in-die targets enablecharacterization of parameter variation within-die. Exemplary parametersof interest include critical dimensions, overlay, and edge placementerrors.

In some embodiments, x-ray illumination source 210, focusing optics 211,slits 212 and 213, or any combination thereof, are maintained in thesame atmospheric environment as specimen 201 (e.g., gas purgeenvironment). However, in some embodiments, the optical path lengthbetween and within any of these elements is long and x-ray scattering inair contributes noise to the image on the detector. Hence in someembodiments, any of x-ray illumination source 210, focusing optics 211,and slits 212 and 213 are maintained in a localized, vacuum environment.In the embodiment depicted in FIG. 14, illumination source 210, focusingoptics 211, and slits 212 and 213 are maintained in a controlledenvironment (e.g., vacuum) within an evacuated flight tube 217. Theillumination beam 214 passes through window 220 at the end of flighttube 217 before incidence with specimen 201.

Similarly, in some embodiments, the optical path length between specimen201 and detector 219 (i.e., the collection beam path) is long and x-rayscattering in air contributes noise to the image on the detector. Hence,in preferred embodiments, a significant portion of the collection beampath length between specimen 201 and detector 219 is maintained in alocalized vacuum environment separated from the specimen (e.g., specimen201) by a vacuum window (e.g., vacuum window 224). In some embodiments,x-ray detector 219 is maintained in the same localized vacuumenvironment as the beam path length between specimen 201 and detector219. For example, as depicted in FIG. 14, vacuum chamber 223 maintains alocalized vacuum environment surrounding detector 219 and a significantportion of the beam path length between specimen 201 and detector 219.

In some other embodiments, x-ray detector 219 is maintained in the sameatmospheric environment as specimen 201 (e.g., gas purge environment).This may be advantageous to remove heat from detector 219. However, inthese embodiments, it is preferable to maintain a significant portion ofthe beam path length between specimen 201 and detector 219 in alocalized vacuum environment within a vacuum chamber.

In some embodiments, the entire optical system, including specimen 201,is maintained in vacuum. However, in general, the costs associated withmaintaining specimen 201 in vacuum are high due to the complexitiesassociated with the construction of specimen positioning system 240.

In another aspect, metrology tool 200 includes a computing system (e.g.,computing system 230) configured to implement beam control functionalityas described herein. In the embodiment depicted in FIG. 14, computingsystem 230 is configured as a beam controller operable to control any ofthe illumination properties such as intensity, divergence, spot size,polarization, spectrum, and positioning of the incident illuminationbeam 214.

As illustrated in FIG. 14, computing system 230 is communicativelycoupled to detector 219. Computing system 230 is configured to receivemeasurement data 235 from detector 219. In one example, measurement data235 includes an indication of the measured response of the specimen(i.e., intensities of the diffraction orders). Based on the distributionof the measured response on the surface of detector 219, the locationand area of incidence of illumination beam 214 on specimen 201 isdetermined by computing system 230. In one example, pattern recognitiontechniques are applied by computing system 230 to determine the locationand area of incidence of illumination beam 214 on specimen 201 based onmeasurement data 235. In some examples, computing system 230communicates command signals 236 to x-ray illumination source 210 toselect the desired illumination wavelength. In some examples, computingsystem 230 communicates command signals 237 to actuator subsystem 215 toredirect the x-ray emission to achieve a desired beam direction. In someexamples, computing system 230 communicates command signals 238 and 239to beam shaping slits 212 and 213, respectively, that cause beam shapingslits 212 and 213 to change the beam spot size and select illuminationwavelengths such that incident illumination beam 214 arrives at specimen201 with the desired beam spot size, orientation, and wavelength(s). Inone example, command signals 238 and 239 cause actuators associated withslits 212 and 213 to change position to reshape the incident beam 214 toa desired shape and size and select desired wavelengths. In some otherexamples, computing system 230 communicates a command signal to waferpositioning system 240 to position and orient specimen 201 such thatincident illumination beam 214 arrives at the desired location andangular orientation with respect to specimen 201.

In a further aspect, RSAXS measurement data is used to generate an imageof a measured structure based on the measured intensities of thedetected diffraction orders. In some embodiments, a RSAXS responsefunction model is generalized to describe the scattering from a genericelectron density mesh. Matching this model to the measured signals,while constraining the modelled electron densities in this mesh toenforce continuity and sparse edges, provides a three dimensional imageof the sample.

Although, geometric, model-based, parametric inversion is preferred forcritical dimension (CD) metrology based on RSAXS measurements, a map ofthe specimen generated from the same RSAXS measurement data is useful toidentify and correct model errors when the measured specimen deviatesfrom the assumptions of the geometric model.

In some examples, the image is compared to structural characteristicsestimated by a geometric, model-based parametric inversion of the samescatterometry measurement data. Discrepancies are used to update thegeometric model of the measured structure and improve measurementperformance. The ability to converge on an accurate parametricmeasurement model is particularly important when measuring integratedcircuits to control, monitor, and trouble-shoot their manufacturingprocess.

In some examples, the image is a two dimensional (2-D) map of electrondensity, absorptivity, complex index of refraction, or a combination ofthese material characteristics. In some examples, the image is a threedimensional (3-D) map of electron density, absorptivity, complex indexof refraction, or a combination of these material characteristics. Themap is generated using relatively few physical constraints. In someexamples, one or more parameters of interest, such as critical dimension(CD), sidewall angle (SWA), overlay, edge placement error, pitch walk,etc., are estimated directly from the resulting map. In some otherexamples, the map is useful for debugging the wafer process when thesample geometry or materials deviate outside the range of expectedvalues contemplated by a parametric structural model employed formodel-based CD measurement. In one example, the differences between themap and a rendering of the structure predicted by the parametricstructural model according to its measured parameters are used to updatethe parametric structural model and improve its measurement performance.Further details are described in U.S. Patent Publication No.2015/0300965, the content of which is incorporated herein by referenceit its entirety. Additional details are described in U.S. PatentPublication No. 2015/0117610, the content of which is incorporatedherein by reference it its entirety.

It should be recognized that the various steps described throughout thepresent disclosure may be carried out by a single computer system 230or, alternatively, a multiple computer system 230. Moreover, differentsubsystems of the system 200, such as the specimen positioning system240, may include a computer system suitable for carrying out at least aportion of the steps described herein. Therefore, the aforementioneddescription should not be interpreted as a limitation on the presentinvention but merely an illustration. Further, the one or more computingsystems 230 may be configured to perform any other step(s) of any of themethod embodiments described herein.

In addition, the computer system 230 may be communicatively coupled tothe x-ray illumination source 210, beam shaping slits 212 and 213,focusing optics actuator system 215, specimen positioning system 240,and detector 219 in any manner known in the art. For example, the one ormore computing systems 230 may be coupled to computing systemsassociated with the x-ray illumination source 210, beam shaping slits212 and 213, focusing optics actuator system 215, specimen positioningsystem 240, and detector 219, respectively. In another example, any ofthe x-ray illumination source 210, beam shaping slits 212 and 113,focusing optics actuator system 215, specimen positioning system 240,and detector 219 may be controlled directly by a single computer systemcoupled to computer system 230.

The computer system 230 may be configured to receive and/or acquire dataor information from the subsystems of the system (e.g., x-rayillumination source 210, beam shaping slits 212 and 213, focusing opticsactuator system 215, specimen positioning system 240, detector 219, andthe like) by a transmission medium that may include wireline and/orwireless portions. In this manner, the transmission medium may serve asa data link between the computer system 230 and other subsystems of thesystem 200.

Computer system 230 of the metrology system 200 may be configured toreceive and/or acquire data or information (e.g., measurement results,modeling inputs, modeling results, etc.) from other systems by atransmission medium that may include wireline and/or wireless portions.In this manner, the transmission medium may serve as a data link betweenthe computer system 230 and other systems (e.g., memory on-boardmetrology system 200, external memory, or external systems). Forexample, the computing system 230 may be configured to receivemeasurement data (e.g., signals 235) from a storage medium (e.g., memory232) via a data link. For instance, spectral results obtained usingdetector 219 may be stored in a permanent or semi-permanent memorydevice (e.g., memory 232). In this regard, the measurement results maybe imported from on-board memory or from an external memory system.Moreover, the computer system 230 may send data to other systems via atransmission medium. For instance, specimen parameter values determinedby computer system 230 may be stored in a permanent or semi-permanentmemory device (e.g., memory 232). In this regard, measurement resultsmay be exported to another system.

Computing system 230 may include, but is not limited to, a personalcomputer system, mainframe computer system, workstation, image computer,parallel processor, or any other device known in the art. In general,the term “computing system” may be broadly defined to encompass anydevice having one or more processors, which execute instructions from amemory medium.

Program instructions 234 implementing methods such as those describedherein may be transmitted over a transmission medium such as a wire,cable, or wireless transmission link. For example, as illustrated inFIG. 14, program instructions stored in memory 232 are transmitted toprocessor 231 over bus 233. Program instructions 234 are stored in acomputer readable medium (e.g., memory 232). Exemplary computer-readablemedia include read-only memory, a random access memory, a magnetic oroptical disk, or a magnetic tape.

In some embodiments, scatterometry measurements as described herein areimplemented as part of a fabrication process tool. Examples offabrication process tools include, but are not limited to, lithographicexposure tools, film deposition tools, implant tools, and etch tools. Inthis manner, the results of a RSAXS analysis are used to control afabrication process. In one example, RSAXS measurement data collectedfrom one or more targets is sent to a fabrication process tool. TheRSAXS measurement data is analyzed as described herein and the resultsused to adjust the operation of the fabrication process tool to reduceerrors in the manufacture of semiconductor structures.

Scatterometry measurements as described herein may be used to determinecharacteristics of a variety of semiconductor structures. Exemplarystructures include, but are not limited to, FinFETs, low-dimensionalstructures such as nanowires or graphene, sub 10 nm structures,lithographic structures, through substrate vias (TSVs), memorystructures such as DRAM, DRAM 4F2, FLASH, MRAM and high aspect ratiomemory structures. Exemplary structural characteristics include, but arenot limited to, geometric parameters such as line edge roughness, linewidth roughness, pore size, pore density, side wall angle, profile,critical dimension, pitch, thickness, overlay, and material parameterssuch as electron density, composition, grain structure, morphology,stress, strain, and elemental identification. In some embodiments, themetrology target is a periodic structure. In some other embodiments, themetrology target is aperiodic.

In general, an x-ray based system employing a multilayer optical elementwith an integrated optical filter as described herein may also includeone or more stand-alone transmittance based optical filters to enhancesuppression of contamination wavelength bands.

In general an integrated optical filter may be located on any opticalelement of an x-ray based system. Although the addition of an integratedoptical filter to a reflective, multi-layer X-ray optical element isdescribed in detail hereinbefore, in general an integrated opticalfilter may be disposed on any optical element in an optical path of anx-ray based system. In some embodiments, an integrated optical filter isfabricated on a detector entrance window (e.g., camera entrance window).In some embodiments, an integrated optical filter is fabricated on anexit window of an x-ray illumination source, or on a collection cavityof the x-ray illumination source. In this manner, an integrated opticalfilter may operate in a transmissive mode, i.e., suppressing selectedwavelengths from radiation passing through the integrated optical filterin a single pass, or in a reflective mode, i.e., suppressing selectedwavelengths from radiation passing through the integrated optical filterin a double pass, depending on the type of optic upon which theintegrated optical filter is fabricated.

In one example, one or more integrated optical filters are included inan optical path of a soft x-ray based metrology system employing a laserproduced plasma (LPP) light source. Typically, the LPP light sourcegenerates harmonics in the IR and visible wavelength ranges. Inaddition, suppression of undesired EUV wavelengths is also desired. Inone example, an integrated optical filter is fabricated on theillumination source window, the detector window, or both, thatsuppresses IR and visible wavelengths. In addition, another integratedoptical filter is fabricated with the multi-layer, reflective x-rayfocusing optics to suppress undesired EUV wavelengths as describedhereinbefore.

In some examples, measurements of critical dimensions, thicknesses,overlay, and material properties of high aspect ratio semiconductorstructures including, but not limited to, spin transfer torque randomaccess memory (STT-RAM), three dimensional NAND memory (3D-NAND) orvertical NAND memory (V-NAND), dynamic random access memory (DRAM),three dimensional FLASH memory (3D-FLASH), resistive random accessmemory (Re-RAM), and phase change random access memory (PC-RAM) areperformed with RSAXS measurement systems as described herein.

As described herein, the term “critical dimension” includes any criticaldimension of a structure (e.g., bottom critical dimension, middlecritical dimension, top critical dimension, sidewall angle, gratingheight, etc.), a critical dimension between any two or more structures(e.g., distance between two structures), and a displacement between twoor more structures (e.g., overlay displacement between overlayinggrating structures, etc.). Structures may include three dimensionalstructures, patterned structures, overlay structures, etc.

As described herein, the term “critical dimension application” or“critical dimension measurement application” includes any criticaldimension measurement.

As described herein, the term “metrology system” includes any systememployed at least in part to characterize a specimen in any aspect,including critical dimension applications and overlay metrologyapplications. However, such terms of art do not limit the scope of theterm “metrology system” as described herein. In addition, the metrologysystems described herein may be configured for measurement of patternedwafers and/or unpatterned wafers. The metrology system may be configuredas a LED inspection tool, edge inspection tool, backside inspectiontool, macro-inspection tool, or multi-mode inspection tool (involvingdata from one or more platforms simultaneously), and any other metrologyor inspection tool that benefits from the measurement techniquesdescribed herein.

Various embodiments are described herein for a semiconductor processingsystem (e.g., an inspection system or a lithography system) that may beused for processing a specimen. The term “specimen” is used herein torefer to a wafer, a reticle, or any other sample that may be processed(e.g., printed or inspected for defects) by means known in the art.

As used herein, the term “wafer” generally refers to substrates formedof a semiconductor or non-semiconductor material. Examples include, butare not limited to, monocrystalline silicon, gallium arsenide, andindium phosphide. Such substrates may be commonly found and/or processedin semiconductor fabrication facilities. In some cases, a wafer mayinclude only the substrate (i.e., bare wafer). Alternatively, a wafermay include one or more layers of different materials formed upon asubstrate. One or more layers formed on a wafer may be “patterned” or“unpatterned.” For example, a wafer may include a plurality of dieshaving repeatable pattern features.

A “reticle” may be a reticle at any stage of a reticle fabricationprocess, or a completed reticle that may or may not be released for usein a semiconductor fabrication facility. A reticle, or a “mask,” isgenerally defined as a substantially transparent substrate havingsubstantially opaque regions formed thereon and configured in a pattern.The substrate may include, for example, a glass material such asamorphous SiO₂. A reticle may be disposed above a resist-covered waferduring an exposure step of a lithography process such that the patternon the reticle may be transferred to the resist.

One or more layers formed on a wafer may be patterned or unpatterned.For example, a wafer may include a plurality of dies, each havingrepeatable pattern features. Formation and processing of such layers ofmaterial may ultimately result in completed devices. Many differenttypes of devices may be formed on a wafer, and the term wafer as usedherein is intended to encompass a wafer on which any type of deviceknown in the art is being fabricated.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by ageneral purpose or special purpose computer. By way of example, and notlimitation, such computer-readable media can comprise RAM, ROM, EEPROM,CD-ROM or other optical disk storage, magnetic disk storage or othermagnetic storage devices, or any other medium that can be used to carryor store desired program code means in the form of instructions or datastructures and that can be accessed by a general-purpose orspecial-purpose computer, or a general-purpose or special-purposeprocessor. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,XRF disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

Although certain specific embodiments are described above forinstructional purposes, the teachings of this patent document havegeneral applicability and are not limited to the specific embodimentsdescribed above. Accordingly, various modifications, adaptations, andcombinations of various features of the described embodiments can bepracticed without departing from the scope of the invention as set forthin the claims.

What is claimed is:
 1. A metrology system, comprising: an x-rayillumination source configured to generate an amount of soft x-rayradiation including multiple illumination wavelengths within a desiredphoton energy range from 80 electronvolts to 3,000 electronvolts and anundesired photon energy range below 80 electronvolts; an x-ray detectorconfigured to detect an amount of x-ray radiation scattered from asemiconductor wafer in response to the amount of soft x-ray radiation; aplurality of x-ray optical elements each having at least one opticalsurface disposed in an optical path between the x-ray illuminationsource and the detector; an integrated optical filter fabricated overthe optical surface of at least one of the plurality of x-ray opticalelements, the integrated optical filter including one or more materiallayers that absorb radiation in the undesired photon energy range andtransmits radiation in the desired photon energy range; and a computingsystem configured to determine a value of a parameter of interestcharacterizing a structure disposed on the semiconductor wafer based onthe detected amount of x-ray radiation.
 2. The metrology system of claim1, wherein the metrology system is a soft x-ray reflectometry system. 3.The metrology system of claim 2, wherein the soft x-ray reflectometrysystem operates in a grazing incidence mode.
 4. The metrology system ofclaim 2, wherein the metrology system operates in an imaging mode. 5.The metrology system of claim 1, wherein the integrated optical filteris disposed over a multilayer x-ray reflecting structure fabricated overthe optical surface of the at least one of the plurality of x-rayoptical elements.
 6. The metrology system of claim 5, furthercomprising: a diffusion barrier layer, the diffusion barrier layerdisposed between the one or more material layers of the integratedoptical filter and the multilayer x-ray reflecting structure, disposedbetween the optical surface and the multilayer x-ray reflectingstructure, or disposed over the one or more material layers of theintegrated optical filter.
 7. The metrology system of claim 1, whereinthe optical surface of the at least one of the plurality of x-rayoptical elements is curved.
 8. The metrology system of claim 1, whereina thickness of the integrated optical filter varies as a function oflocation on the optical surface of the at least one of the plurality ofx-ray optical elements.
 9. The metrology system of claim 1, furthercomprising: a stand-alone optical filter disposed in the optical pathbetween the x-ray illumination source and the detector.